1. Field of the Invention
The present invention relates to a plasma display device, and more particularly, to a driving circuit of a plasma display device for implementing gray scales by performing scanning in a sub-frame method after dividing a frame into at least two subfields.
2. Description of the Related Art
As the current society has become more information-oriented, the desire for development and distribution of information processing systems has recently increased. Thus, the importance of picture display devices has become increased and the kinds thereof has become gradually varied.
Conventional CRTs (cathode-ray tubes) which have been widely used have several problems such as large size, high operational voltage, display distortion and the like. Thus, they are not suitable for the demand for realization of a large-scale and flat screen. Currently, various flat display devices having matrix structures are being studied and developed.
Among the flat display devices, a plasma display device having an alternating current plasma display panel (AC PDP) which is an emitting type element, displays motion pictures or still pictures by using a gas discharge phenomenon of the AC PDP.
In the plasma display device, discharge is achieved by adjusting voltages between vertical and horizontal electrodes of a cell composing a pixel. The amount of discharged light changes to adjust the length of discharge time in the cell. The overall screen is obtained by driving in a matrix type a write pulse for inputting a digital picture signal to vertical and horizontal electrodes of the respective cells, a scan pulse for scanning, a sustain pulse for sustaining discharge, and an erase pulse for terminating discharge of a discharged cell. Also, a gray scale is implemented by differentiating the length of discharge time of each cell for a predetermined time (e.g., 1/30 sec in an NTSC Television signal) required for displaying the entire picture.
The luminance of a screen is determined by the brightness for the case when each cell is driven to a maximum level. To increase the luminance, a driving circuit must be constructed such that the discharge time of a cell can be maintained as long as possible for a predetermined time required for forming a screen. Contrast which is a difference in light and darkness is determined by brightness and luminance of a background such as illumination. To increase the contrast, the background must be dark and the luminance thereof must be increased.
FIG. 1 is a structural view of an electrode of an AC PDP included in a conventional plasma display device, in which M vertical electrode lines D.sub.1 to D.sub.M and N horizontal electrode lines S.sub.1 to S.sub.N.
In the AC PDP, the corresponding cell is discharged such that voltages applied between a vertical electrode line and a horizontal electrode line are adjusted. Also, the gray scale of a picture displayed in each cell is implemented by adjusting the discharge sustaining time of the cell.
As a method for implementing the gray scale of the displayed picture, there is a subfield method. According to the gray scale implementing method using the subfield method, a frame is divided into X subfields and a luminance value proportional to a relative luminance ratio (1:2:4:8:16:32:64: . . . ) is allotted to each subfield to then display a picture of 2.sup.X gray levels by the combination of the respective subfields, which is disclosed in U.S. Pat. No. 5,541,618.
For example, as shown in FIG. 2, one frame is divided into four subfields sf1, sf2, sf3 and sf4, and the corresponding gray scale data are supplied to the entire cells for each subfield. Then, if sustain pulses of the number proportional to the relative luminance ratio 1:2:4:8 are supplied, the picture of 2.sup.4 (=16) gray scale levels is displayed.
In other words, if one sustain pulse is supplied when a first subfield sf1 is driven, two, four and eight sustain pulses are supplied, respectively, when second to fourth subfields sf2 to sf4 are driven.
However, the above-described gray scale implementing method using the subfield method has limitation in that a write pulse cannot be applied to one or more horizontal electrodes once with respect to a given vertical electrode since the PDP must be driven by a matrix method. Accordingly, horizontal electrodes must be driven at different timings from each other. Therefore, a time for scanning all horizontal electrodes is required for forming the respective subfields. The respective cells can sustain discharge only for a reduced time by a scanning time from the average time allotted to the respective subfields. At this time, the time required for scanning is increased as the number of horizontal electrodes are increased. Since the discharge cannot be sustained for the time, the luminance and contrast of the PDP are lowered. Also, in forming subfields, a discharge time difference between upper and lower bits is large. Also, a plurality of subfields are sequentially driven for displaying a picture of one frame. In this case, since a procedure of maintaining discharge emission during a relevant period by discharge and emitting the corresponding cell after erasing the entire cells every driving time of each subfield, a flicker is generated due to the discharge time difference.
To solve the problems caused in the subfield method, research into a method for implementing a gray scale using a subframe method corresponding to the subfield method are being actively made.
According to the gray scale implementing method using the sub-frame method, the total horizontal lines of one frame are divided into X blocks depending on a relative luminance ratio so that horizontal lines of the number proportional to the relative luminance ratio may belong to each block. Then, while each X horizontal lines from the first horizontal line to the one right before the last horizontal line, included in each block, are sequentially scanned at a time, the corresponding gray scale data are supplied, thereby displaying a picture of 2 .sup.X gray levels. At this time, each block is called a subframe.
For example, as shown in FIG. 3, assuming that the total horizontal electrode lines of one frame are 15, one frame is divided into four subframes SF1, SF2, SF3 and SF4 according to a relative luminance ratio 1:2:4:8. Eight horizontal lines 1 to 8, four horizontal lines 9 to 12, two horizontal lines 13 and 14 and one horizontal line 15 are included in the first, second, third and fourth subframes SF1, SF2, SF3 and SF4, respectively.
As described above, in a state where one frame is divided into four subframes SF1 to SF4, the first subframe SF1 sequentially scans each four horizontal lines, from the horizontal line 1 (S.sub.1) to the horizontal line 15 (S.sub.15), at a time, the second sub-frame SF2 from the horizontal line 9 (S.sub.9) to the horizontal line 8 (S.sub.8), the third subframe SF3 from the horizontal line 13 (S.sub.13) to the horizontal line 12 (S.sub.12), and the fourth subframe SF4 from the horizontal line 15 (S.sub.15) to the horizontal line 14 (S.sub.14). In the meanwhile, if the corresponding gray scale data are supplied, a display picture of 2.sup.4 (=16) gray scale levels is implemented.
A driving circuit of a plasma display device for implementing 16 gray scale levels using the sub-frame method, as shown in FIG. 4, includes a microprocessor (to be referred to as a micom, hereinafter) 120 for digitizing analog picture data to output digital picture data and outputting various control signals according to the digital picture data and external signals, a scanning and sustaining driver 130 for sequentially erasing each four lines for every 1/15 frame (to be referred to as one horizontal period, hereinafter) from the first horizontal lines to the last fifteenth ones, included in each four subframes SF1, SF2, SF3 and SF4, scanning the same, and supplying sustain pulses for sustaining discharge to the scanned horizontal electrode lines, a memory 140 for storing the digital picture data output from the micom 120 by frames, colors and bits, and an address driver 150 for receiving bit values of 20 digital picture data corresponding to the horizontal electrode line currently scanned by the scanning and sustaining driver 130 from the memory 140 and supplying the same to 20 vertical electrode lines D.sub.1 to D.sub.20 formed in an AC PDP 10, respectively.
The driving circuit of the plasma display device having the aforementioned configuration operates as follows.
First, the micom 120 digitizes externally input analog picture data to then output 4-bit digital picture data B.sub.1 to B.sub.4 and outputs various control signals according to the digital picture data B.sub.1 to B.sub.4 and external signals.
At this time, the digital picture data output from the micom 120 are stored in the memory 140 by frames, colors and bits.
Thereafter, according to the control signals output from the micom 120, the scanning and sustaining driver 130 repeatedly turns the entire screen of the AC PDP 10 on and off, thereby forming a wall charge within the discharge space of each cell. Then, each four lines from each first horizontal electrode lines S.sub.1, S.sub.9, S.sub.13 and S.sub.15 to the last fifteenth horizontal electrode lines S.sub.15, S.sub.8, S.sub.12 and S.sub.14, included in the first to fourth subframes SF1 to SF4, respectively, are sequentially erased and then scanned.
The address driver 150 supplies to 20 vertical electrode lines D.sub.1 to D.sub.20 bit values of 20 digital picture data corresponding to the horizontal electrode line currently scanned by the scanning and sustaining driver 130, thereby displaying a picture of 16 (=2.sup.4) gray scale levels on the AC PDP 10.
At this time, the scanning and sustaining driver 130 sequentially scans each four horizontal electrode lines with a predetermined time interval for every horizontal period. Also, the scanning and sustaining driver 130 scans the respective horizontal electrode lines S.sub.1 to S.sub.15 and then supplies sustain pulses, so that a logic `high` signal is supplied through the corresponding vertical electrode line among 20 cells corresponding to the respective horizontal electrode lines S.sub.1 to S.sub.15. Thus, the discharge and emission of partial cells experiencing address discharge are sustained for a predetermined time.
The scanning sequence of the respective horizontal electrode lines S.sub.1 to S.sub.15 is tabulated in the following table 1.
TABLE 1 __________________________________________________________________________ Horizontal period 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 __________________________________________________________________________ SF1 S.sub.1 S.sub.2 S.sub.3 S.sub.4 S.sub.5 S.sub.6 S.sub.7 S.sub.8 S.sub.9 S.sub.10 S.sub.11 S.sub.12 S.sub.13 S.sub.14 S.sub.15 SF2 S.sub.9 S.sub.10 S.sub.11 S.sub.12 S.sub.13 S.sub.14 S.sub.15 S.sub.1 S.sub.2 S.sub.3 S.sub.4 S.sub.5 S.sub.6 S.sub.7 S.sub.8 SF3 S.sub.13 S.sub.14 S.sub.15 S.sub.1 S.sub.2 S.sub.3 S.sub.4 S.sub.5 S.sub.6 S.sub.7 S.sub.8 S.sub.9 S.sub.10 S.sub.11 S.sub.12 SF4 S.sub.15 S.sub.1 S.sub.2 S.sub.3 S.sub.4 S.sub.5 S.sub.6 S.sub.7 S.sub.8 S.sub.9 S.sub.10 S.sub.11 S.sub.12 S.sub.13 S.sub.14 __________________________________________________________________________
The address driver 150 supplies the least significant bit B.sub.1 among corresponding 20 digital picture data to the vertical electrode lines D.sub.1 to D.sub.20 while 15 horizontal electrodes lines S.sub.1 to S.sub.15 are sequentially scanned to the first subframe SF1, supplies the most significant bit B.sub.4 while 15 horizontal electrodes lines S.sub.9 to S.sub.8 are sequentially scanned to the second subframe SF2, supplies the third bit B.sub.3 while 15 horizontal electrodes lines S.sub.15 to S.sub.14 are sequentially scanned to the third subframe SF3, and supplies the second bit B.sub.2 while 15 horizontal electrodes lines S.sub.15 to S.sub.14 are sequentially scanned to the fourth subframe SF4.
Now, in the case of the horizontal electrode line S.sub.1, for example, the bit values of the digital picture data supplied to the respective horizontal electrodes lines S.sub.1 to S.sub.15 during one frame (1 to 15 horizontal periods) will be explained. From the above Table 1, the horizontal electrode line S.sub.1 is scanned during the horizontal periods 1, 2, 4 and 8, respectively, so that 4-bit digital picture data are supplied in the unit of bits. During the horizontal period 1, the bit B.sub.1 having a weight value 2.sup.0 (=1) is supplied so that emission is maintained during the horizontal period 1 until the horizontal electrode line S.sub.1 is scanned during the horizontal period 2. During the horizontal period 2, the bit B.sub.2 having a weight value 2.sup.1 (=2) is supplied so that emission is maintained until the horizontal electrode line S.sub.1 is scanned during the horizontal period 4. During the horizontal period 4, the bit B.sub.3 having a weight value 2.sup.2 (=4) is supplied so that emission is maintained until the horizontal electrode line S.sub.1 is scanned during the horizontal period 8. During the horizontal period 8, the bit B.sub.4 having a weight value 2.sup.4 (=16) is supplied so that discharge is maintained until the horizontal electrode line S.sub.1 is scanned during the horizontal period 1 of the next frame. Therefore, the picture of 16 gray scale levels is displayed on 20 cells corresponding to the horizontal electrode line 1 S.sub.1.
Different sustaining periods of the gray scale data are also true of the horizontal electrodes 2 to 15 S.sub.2 to S.sub.15, so that the picture of 16 gray scale levels is displayed on 20 cells corresponding thereto, respectively.
Also, in the case of implementing 2.sup.8 (=256) gray scales using the sub-frame method, while 8 horizontal electrode lines are sequentially scanned during the horizontal period 1, the corresponding gray scale data must be supplied. Further, in the case of implementing 2.sup.16 (=65536) gray scales using the sub-frame method, while 16 horizontal electrode lines are sequentially scanned during the horizontal period 1, the corresponding gray scale data must be supplied. At this time, in the case of a motion picture, the time required for displaying the picture of one frame (to a narrow sense, one horizontal period for scanning 16 horizontal electrode lines) is all the same irrespective of gray scales to be implemented. Thus, the scanning frequency for 65536 gray scale implementation is double that for 256 gray scale implementation. In other words, the more the gray scales to be implemented, the more the horizontal electrodes lines must be scanned during one horizontal period. Thus, if a motion picture is supplied externally, as the gray scales to be implemented become more, the scanning frequency of horizontal electrode lines is increased.
Therefore, the sub-frame method has the following shortcomings. Since the scanning frequency of horizontal electrode line is increased according to the increase in gray scales, the system becomes destabilized and power consumption is increased. Also, since the number of horizontal electrode lines scanned by a scanning and sustaining driver during one horizontal period is limited, it is difficult to realize gray scales more than 256.